Regulation of IgM antibody responses to several bacterial lipopolysaccharides, including Escherichia coli 0113 LPS (EC 0113-LPS), E.coli 055 LPS (EC 055-LPS) and Serratia marcesans LPS (SM-LPS) was examined. Memory to EC 0113-LPS was generated after exposure to a single subimmunogenic dose of this antigen; it developed in a cyclic manner and persisted for at least 20 days. By contrast, mice primed with a subimmunogenic dose of EC 055 LPS or SM-LPS developed profound suppression rather than memory for the antobody response to the same LPS used for priming; the development of such suppression was dose and time dependent and not due to a switch in immunoglobulin isotype. The cellular mechanisms involved in the expression of memory and suppression to LPS are being investigated in detail. The results of other studies showed that polyclonal stimulation by LPS results in the preferential activation of memory, rather than virgin B cells.